This invention relates to magnetic amplifiers, and more particularly to a low level DC transducer which operates on magnetic amplifier principles.
Magnetic amplifiers are ideally suited to the task of amplifying very low DC signals since a small current in a control winding can have a large effect in the load current. One application for such a very low level amplifier is monitoring strategic points in complex systems, such as power conditioning systems of spacecraft for the purpose of effecting judicious management of power.
Measurement of DC current flow in a circuit usually requires insertion of a DC meter in series with the source and load. In applications involving high voltages this may be undesirable because of the lack of isolation. In other instances, power wasting meter shunt-resistors may be needed. These disadvantages can be avoided by use of the so-called "DC transformer" arrangement which operates on magnetic amplifier principles. Instead of the classic arrangement in which a small DC current controls the flow of a large amount of AC power, a constant amplitude, fixed frequency AC excitation is applied to one winding of a transformer having a core which exhibits a desired saturation characteristic. The DC current flow which is being monitored is fed into another winding of the transformer to produce a magnetic core flux which varies in accordance with the DC current flow. Since circuit reactance is a function of the degree of saturation of the core produced by the DC current flow, the output of the transformer (usually rectified for use by a DC meter) indicates the DC current flow, which sometimes may be at a very low level.
Magnetic amplifiers used as DC current transducers heretofore have been relatively bulky and heavy. As a consequence, their use has been restricted to a relatively small number of observation points in the power conditioning system of space craft. Transducers on recent spacecraft weighed 100 grams. That weight has been reduced to 60 grams, and more recent effort has further reduced that weight to only 4 grams. Such a great reduction in weight will permit more extensive use on spacecraft, and in other applications as well since bulk and weight are significant factors even in commercial applications.
With the possibility of being able to monitor currents at more points in a system, it becomes desirable to monitor points which normally experience very low currents of less than 0.1 ampere. The circuit designs for transducers of larger DC currents have presented no problem in the miniaturizing program. This is because magnetic amplifiers used as DC current transducers easily exhibit a relatively linear transfer function in the large current region. There is a region below 0.1 ampere in which the transducer exhibits a large inverse function, i.e., a transfer function in which the output increases at a high rate as the DC input current decreases at lower rate. Consequently, a conventional magnetic amplifier circuit can not be used for monitoring such low DC currents. What is needed is a circuit design which exhibits a substantially linear transfer function from relatively high DC currents of about 0.5 amperes down below 0.1 amperes to no DC current.